Norns. DEVELOPMENT OF MARINE SPONGES. 281 
(the so-called subdermal cavities, s.d.c., Pl. XIV, Fig. 2), separates 
more or less completely the superficial layer of the sponge from 
the rest of the body. This superficial layer (ectoderm + thin 
layer of mesoderm) is known as the dermal membrane, @. mem, 
Fig. 2. The dermal membrane is bound to the subjacent part 
of the body by the mesoderm lying between the subdermal 
cavities. The cavities are so numerous that this mesoderm 
takes the form of irregular beams or trabeculae, often being 
nothing more than slender cords, as in Fig. 4, mes. 6. The 
dermal membrane may easily be pulled off, and is then found 
to be everywhere perforated by closely set pores, Fig. 4 (surface 
view of dermal membrane, from below). The subdermal cavities 
into which the pores open are of two kinds, of which the larger 
and deeper appear as pore areas (often rounded) on the surface. 
These pore areas are conspicuous when the surface of the 
sponge is examined with a lens, and the dermal membrane 
between them seems at first sight not to possess any pores 
(Fig. 5, view of a small area of the surface). This is owing to 
the fact that the second kind of subdermal cavity, underlying 
the apparently aporous portion of the dermal membrane, is very 
small and shallow. That the dermal membrane does possess 
pores between the pore areas can easily be shown by scraping 
the membrane free of the subjacent tissue, when the pores at 
once come into view (region a in Fig. 5). Owing to the great 
number of pore areas the surrace of the sponge acquires a 
characteristically porous appearance. When examined under a 
low objective or magnifying glass the surface is further diver- 
sified by an irregular meshwork of dark bands, which represent 
the coarser mesodermic trabeculae connecting the dermal mem- 
brane with the body of the sponge (Fig. 5). 
The water passing through the pores enters the subdermal 
cavities, s.d.c., Fig. 2, whence it may pass by afferent canals, 
a.f.c., to the flagellated chambers. - The flagellated chambers 
are arranged along the sides of the afferent canals, and 
open directly into them. At their opposite pole the 
chambers open in the same way into efferent canals (ef c.). 
Both afferent and efferent canals are relatively large. There 
is no great difference in size between the afferent canals 
